Many vessels in animals transport fluids from one bodily location to another. Frequently, fluid flows in a substantially unidirectional manner along the length of the vessel. For example, veins in the body transport blood to the heart and arteries carry blood away from the heart.
In mammalian veins, natural valves are positioned along the length of the vessel in the form of leaflets disposed annularly along the inside wall of the vein which open to permit blood flow toward the heart and close to restrict back flow. These natural venous valves open to permit the flow of fluid in the desired direction, and close upon a change in pressure. When blood flows through the vein, the pressure forces the valve leaflets apart as they flex in the direction of blood flow and move towards the inside wall of the vessel, creating an opening therebetween for blood flow. When the pressure differential across the valve, the flow velocity, or both change, the leaflets return to a closed position to restrict or prevent blood flow in the opposite, i.e. retrograde, direction. The leaflet structures, when functioning properly, extend radially inwardly toward one another such that the tips contact each other to restrict backflow of blood.
In the condition of venous insufficiency, the valve leaflets do not function properly. Incompetent venous valves can result in symptoms such as swelling and varicose veins, causing great discomfort and pain to the patient. If left untreated, venous insufficiency can result in excessive retrograde blood flow through incompetent venous valves, which can cause venous stasis ulcers of the skin.
There generally are two types of venous insufficiency: primary and secondary. Primary venous insufficiency typically occurs where the valve structure remains intact, but the vein is simply too large in relation to the leaflets so that the leaflets cannot come into adequate contact to prevent backflow. More common is secondary venous insufficiency, where the valve structure is damaged, for example, by clots which gel and scar, thereby changing the configuration of the leaflets, i.e. thickening the leaflets and creating a “stub-like” configuration. Venous insufficiency can occur in the superficial venous system, such as the saphenous veins in the leg, or in the deep venous system, such as the femoral and popliteal veins extending along the back of the knee to the groin.
A common method of treatment of venous insufficiency is placement of an elastic stocking around the patient's leg to apply external pressure to the vein. Although sometimes successful, the tight stocking is quite uncomfortable, especially in warm weather, as the stocking must be constantly worn to keep the leaflets in apposition. The elastic stocking also affects the patient's physical appearance, thereby potentially having an adverse psychological affect. This physical and/or psychological discomfort can lead to the patient removing the stocking, thereby preventing adequate treatment.
Surgical methods for treatment of venous insufficiency have also been developed. A vein with incompetent venous valves can be surgically constricted to bring incompetent leaflets into closer proximity in an attempt to restore natural valve function. Methods for surgical constriction of an incompetent vein include implanting a frame around the outside of the vessel, placing a constricting suture around the vessel, or other types of treatment of the outside of the vessel to induce vessel contraction. Other surgical venous insufficiency treatment methods include bypassing or replacing damaged venous valves with autologous sections of veins with competent valves. However, these surgeries often result in a long patient recovery time and scarring, and carry the risks, e.g. anesthesia, inherent with surgery.
Recently, various implantable prosthetic devices and minimally invasive methods for implantation of these devices have been developed to treat venous insufficiency, without the disadvantages of treatment with an outer stocking or extensive surgery. Such prosthetic venous valve devices can be inserted intravascularly, for example from an implantation catheter. Prosthetic devices can function as a replacement valve, or restore native valve function by bringing incompetent valve leaflets into closer proximity or by modifying the flow through the vessel.
Valves for implantation into veins are known in the art having leaflets positional to allow fluid flow through a vessel in a first direction and to restrict fluid flow through the vessel in a second direction. Examples of such prosthetic valves are described in commonly owned U.S. Pat. No. 7,153,324 and U.S. Publication No. 2004/0260389.
It is desirable to have prosthetic devices for modifying fluid flow through a vessel that permit fluid flow in a first direction and wherein the devices are at least partially collapsible in a second direction to modify fluid flow therethrough.